I use the technique you're describing a fair bit in practice but it's never really worked for me in competition for some reason. You basically do a ko-uchi and hold it while reaching down and grabbing the ankle. If you drop to one knee (the outside one) and shove them backwards with your shoulder, they are going down since you've completely frozen that leg. That'll leave you in a kind of outside half guard which isn't too bad of a position to work out of (in judo, BJJ, MMA).